UV‐irradiation influence on free radical formation and radical scavenging ability of caffeic acid—EPR, UV‐Vis, and colorimetric examination

Abstract Electron paramagnetic resonance (EPR) spectroscopy, UV‐Vis spectrophotometry, and colorimetric analysis, were used to examine nonirradiated and UV‐irradiated caffeic acid. In this work free radical formation during UV‐irradiation of caffeic acid and the effect of UV‐irradiation on its radical scavenging ability, were studied. The influence of time of UV‐irradiation on these problems was evaluated. EPR spectra of free radicals in caffeic acid and 1,1‐diphenyl‐2‐picrylo‐hydrazyl (DPPH) were measured. DPPH was used as the model free radical reference to test antioxidative properties of caffeic acid. Amplitudes, linewidths, integral intensities, and g‐factors were analyzed. Free radical concentrations were determined by comparing of the EPR spectra of caffeic acid and ultramarine—the paramagnetic reference. Free radicals were found (~10 18 spin/g) in all the UV‐irradiated samples independent on irradiation time. Free radical concentrations in caffeic acid increased with increasing of irradiation time. Analysis of spectral shape for different microwave powers pointed out that free radicals system in UV‐irradiated caffeic acid was complex. Nonirradiated and UV‐irradiated caffeic acid as antioxidants strongly interacted with free radicals and EPR signals of DPPH were highly quenched. UV‐irradiation increased interactions of caffeic acid with free radicals, and this effect increased with increasing of irradiation time. Practical Application The performed studies indications that caffeic acid should not be storage exposed to UV. UV‐irradiation formed free radicals in caffeic acid and changed its antioxidative properties as radical scavenging ability. Free radical concentrations in caffeic acid increased with increased UV irradiation time. UV‐irradiation causes a change of color in caffeic acid. The change of color and formation of free radicals in caffeic acid during UV‐irradiation can indicate the destruction of the chemical structure of tested samples.